TIMING OF METAMORPHISM IN THE TAUERN WINDOW, EASTERN ALPS - RB-SR AGES AND FABRIC FORMATION

The Peripheral Schieferhulle of the Tauern Window of the Eastern Alps represents post-Hercynian Penninic cover sequences and preserves a rec ord of metamorphism in the Alpine orogeny, without the inherited remna nts of Hercynian events that are retained in basement rocks. The tempe rature-time-deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrogr aphic study of units whose P-T evolution and structural setting are al ready well understood. The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alp ine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase , associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the e clogite zone and cover schists, recording deformation and peak metamor phism at 28-30 Ma. The Peripheral Schieferhulle of the south-east Taue rn Window yields Rb-Sr white mica ages which can be tied to the struct ural evolution of the metamorphic pile. Early prograde fabrics pre-dat e 31 Ma, and were reworked by the formation of the large north-east ve rgent Sonnblick fold structure at 28 Ma. Peak metamorphism post-dated this deformation, but by contrast to the equivalent levels in the cent ral Tauern, peak metamorphic conditions did not lead to widespread hom ogenization of the Sr isotopes. Localized deformation continued into t he cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples. These isotopic ages may be integrate d with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. How ever, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.